1. Field of the Invention
The present invention relates to an image forming apparatus which employs an electrophotographic system.
2. Description of the Related Art
As an image forming apparatus employing the electrophotographic system, there is an image forming apparatus which uses an intermediate transfer belt. Such an image forming apparatus is provided with photosensitive drums serving as image bearing members as many as colors (for example, four colors) necessary for forming an image. Each photosensitive drum is provided with a charging portion, an exposing portion, and a developing portion in the vicinity thereof.
Then, after a monochrome toner image formed on the photosensitive drum is primarily transferred onto the intermediate transfer belt, the toner image primarily transferred on the intermediate transfer belt is secondarily transferred onto a sheet, so that an unfixed image is formed. The sheet with the unfixed image formed thereon is conveyed to the fixing portion to fix the unfixed image, and then discharged from the image forming apparatus.
The image forming apparatus is desirable to support a wide variety of sheets such as in size and in basis weight. However, an image forming apparatus in the related art has a problem in that when the toner image is secondarily transferred onto a thin sheet having a basis weight of 52 g/m2 or less or a sheet having a low stiffness (strength of the middle portion), the sheet may be wound up without being separated from the outer peripheral surface of the intermediate transfer belt.
The sheet wound up in the outer peripheral surface of the intermediate transfer belt may enter places other than a sheet conveying path, so that there is a possibility to make a jam processing operation difficult or to cause trouble in the image forming apparatus.
In Japanese Patent Laid-Open No. 11-59962, a sheet detection sensor is provided on a downstream side in a sheet conveying direction of a secondary transfer portion in order to detect whether the leading end of the sheet is wound up in the outer peripheral surface of the intermediate transfer belt. Further, there is proposed a technology in which when it is determined that there is no sheet on the sheet conveying path, driving operations of the intermediate transfer belt and other conveying rollers are stopped.
Other portions of the sheet besides the leading end may be wound up to the outer peripheral surface of the intermediate transfer belt. For example, when a jam occurs in the sheet, and in a case where a driving portion of the intermediate transfer belt is stopped in a state where the sheet is interposed in the secondary transfer portion, the driving portion of the intermediate transfer belt rotates in some degree by its own inertia even after the driving portion is stopped.
Therefore, a loop is formed in the sheet between the secondary transfer portion and a sheet conveying portion which is provided on the sheet conveying path from the secondary transfer portion to the fixing portion. Then, the intermediate transfer belt is stopped in a state where the middle portion of the sheet with the loop formed thereon is wound up to the outer peripheral surface of the intermediate transfer belt.
In this state, as described in Japanese Patent Laid-Open No. 11-59962, it is not possible to detect the sheet using the sheet detection sensor which is provided on the downstream side in the sheet conveying direction of the secondary transfer portion. In a case where the next printing job starts without processing the jam of the sheet in this state, the sheet in the wound state is conveyed to the outer peripheral surface of the intermediate transfer belt, so that there is a possibility for the sheet to enter places other than the sheet conveying path.
The invention has been made in order to solve the above problem, and it is desirable to provide an image forming apparatus which can prevent a sheet jam caused by a sheet attached to the outer peripheral surface of the image bearing member.